This invention relates to coupling between an outer antenna and a radio receiver having a bar antenna, and, in particular, to a method and devices for coupling an optionally used outer antenna with the receiver.
A bar antenna which comprises a magnetic bar core and a loop antenna coil wound on the bar core has been advantageously used in radio receiver, because it is small in the volume and is able to be assembled into or onto receiver cabinets.
But, in the reception in buildings, the good reception of broadcasts is not enjoyed, because the field intensity of broadcast waves is low and because the receiver is subjected to various noise.
To improve the reception in buildings, the bar antenna 1 has been provided with a link coil 3 wound on the bar core thereof, in addition to the loop antenna coil 2, to enable the use of an outer antenna, for example, a rod or whip antenna 4 by connecting the outer antenna to the link coil, as shown in FIG. 1.
But there are some disadvantages in the use of the rod antenna by connecting the rod antenna to the link coil.
A disadvantage is that the receiving sensibility lowers when a signal received at the bar antenna is in anti-phase with another signal received at the rod antenna.
Since the bar antenna is a loop antenna, the directivity of the bar antenna is as shown in FIG. 2a. On the other hand, the rod antenna is non-directional as shown in FIG. 2b. Accordingly, the resultant directivity is a cardioid characteristic as shown in FIG. 2c. Therefore, it will be noted that the intensity of the resultant wave signal in the use of the rod antenna and the bar antenna is rather lower than the use of the bar antenna alone, depending on the orientation of the bar antenna. It is, of course, achieved by changing the orientation of the radio receiver to obtain a greater resultant receiving power. But the change of the receiver orientation is inconvenient in actual use.
Another disadvantage is that the signal to noise ratio (S/N) lowers in a certain case.
In radio receivers using a commercial AC power, AC power lines are grounded through capacitors 5 and 5' as shown in FIG. 1, to remove noise which enters through the AC power lines. Sometimes, stray capacities between windings and core of a power transformer are employed in place of the capacitors. While, the link coil 3 is grounded at one terminal thereof. Accordingly, the noise which was removed through capacitors 5 and 5', is transmitted to the rod antenna 4 through the earth line and the link coil 3, and presents together with a signal received at the rod antenna 4 in a tuning circuit comprising the antenna coil 2 and a variable condenser.
Another disadvantage is that the rod antenna is not so effectively used.
Assuming that the signal to noise ratio (S/N) at the bar antenna is 1/0.1=10 (20 dB), and that the S/N at the rod antenna is 1/0.01=100 (40 dB), the S/N of the resultant signal is (1+1)/(0.1+0.01).apprxeq.2/0.1=20 (26 dB). This teaches that the use of the rod antenna in addition to the bar antenna improves the S/N only by 6 dB. The resultant S/N is far lower than that of the rod antenna alone.
It is another disadvantage that the additional rod antenna is used in a state impedance-mismatched with the link coil, because the link coil is fixedly mounted on the bar antenna, if the additional rod antenna is not correctly selected.